Two-phase transformer and welding circuit therefor

ABSTRACT

An apparatus for converting three-phase electrical energy into direct current includes Scott-connected transformers with oppositely biased SCRs connected in parallel to two of three input terminals. The SCRs are controlled so that they fire in alternate and rotating sequence between the two input terminals. The secondary output voltages of four terminals are rectified so that the common rectified output voltage is a substantially DC voltage with a minimum ripple. The secondary output terminals are aligned in a common plane and are connected to deformable, tapered leads. The rectifiers are connected between the deformable leads and to a common slotted, conductive plate which is connected to a welding electrode. A center tap from each of the secondary windings is connected to another welding electrode. The deformable tapered leads and the slotted conductive plate provide an equal resistive path for pairs of rectifiers for each secondary output to equalize the current flow through the two sets of diodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to converting alternating current to directcurrent. In one of its aspects, the invention relates to a resistancewelding apparatus utilizing Scott-connected transformers for convertingthree-phase alternating current to direct current.

2. State of the Prior Art

DC welding systems are typically provided by converting three-phaseelectrical energy to direct current through a three-phase transformerwith rectified outputs. For example, see U.S. Pat. Nos. 2,221,576 toDawson (issued Nov. 12, 1940), Aldenhoff 3,339,107 (issued Aug. 29,1967), Riley 3,460,022 (issued Aug. 5, 1969) and Wolf 3,840,720 (issuedOct. 8, 1974). Such three-phase systems are bulky and require expensivecontrols to control the firing input or output voltages to produce asmooth output voltage pattern with substantially constant conduction andminimum ripple effect.

Sciaky, in U.S. Pat. No. 3,495,067 (issued Feb. 10, 1970), discloses theuse of Scott-connected transformers to convert three-phase alternatingcurrent to direct current. Conductive coils are provided in thesecondary circuit for adjusting the inductance of the conductors in thesecondary circuit so as effect the correct commutation of current fromone phase to the next in the primary. This system is expensive,cumbersome and, without a precise timed primary is somewhat inaccurateas a balancing means.

In welding apparatus, the secondary output terminals from thetransformers are typically connected to diode assemblies which rectifythe output current. The direct current terminals of the diodes areconnected together so that the common output from the diodes has arelatively constant DC voltage during the welding cycle. In some cases,multiple diodes are connected to the secondary output terminals.Different conductor lengths may result in unequal current division amongthe rectifier cells in any given array. Riley, in U.S. Pat. No.3,460,022 (issued Aug. 5, 1969), discloses the use of a looped ACconnector in the form of a rectangular frame to enhance equal currentdivision among diode cells. Further, slots are used in the frame tofurther enhance current division along any given row of such diodecells. Flexible braided leads are provided between the diodes and therectangular frame. These braided leads have limited current carryingcapacity. Therefore, multiple diodes must be used when these braidedconnectors are used.

SUMMARY OF THE INVENTION

According to the invention, unique control means are provided forregulating the voltage supply to first and second primary inputterminals of Scott-connected transformers for converting three-phasealternating current to direct current. Two primary windings of theScott-connected transformers have three input terminals for connectingto a three-phase power supply, a pair of secondary windings with outputterminals at opposite ends thereof and a center tap output terminalconnected to the center of each secondary winding. Means connecting thesecondary output terminals together rectify the output voltagestherefrom to produce a substantially constant direct current voltage.

The control means provides for the application of a voltage to the firstand the second primary terminals alternately through each 90° phase ofthe electrical cycle. The first input terminal is connected to one endof one primary winding and a second input terminal is connected to anend of the other primary winding. The third input terminal is connectedto an opposite end of the first mentioned primary winding. Thethree-phase power supply is connected to the input terminals such thatthe phase rotation is from first to the third to the second inputterminal. By this arrangement, the rectified output from the rectifyingmeans is substantially conductive throughout the entire electrical cycleand the ripple effect due to transformation of alternating to directcurrent is minimized.

The control means includes first and second oppositely biased, gatedrectifying means connected in parallel to the first input terminal and athird and fourth oppositely biased, gated rectifying means connected inparallel to the second input terminal. Means are further provided forgating the first rectifier means during a first 90° time interval, forgating the third rectifying means during a second 90° time interval, forgating the second rectifying means during a third 90° time interval, andfor gating the fourth rectifying means during a fourth 90° timeinterval. Desirably, the gated rectifying means are silicon controlledrectifiers or ignitron tubes. The control means eliminates the need forinductive reactances in the secondaries.

Also, according to the invention, deformable leads are connected betweeneach of the secondary output terminals and the rectifying means. Acommon conductor plate member is provided and means mount the rectifyingmeans between the common conductor plate and the deformable leads sothat the leads can respond to any thickness variations of the diodes forapplication of equal pressure to each diode. To this end, the leads areformed from laminated thin straps of conductive material. Preferably,two or more diodes are provided between each lead and conductor plateand the leads have a first thickness adjacent the output terminals and asecond thickness substantially less than the first thickness at thesecond diode and remote from the secondary output terminals. Further,the conductor plate is slotted between the two sets of diodes to providean equal resistive current path so that the voltage drop across bothsets of diodes is substantially equalized.

Desirably, the secondary output terminals are positioned in a commonplane to facilitate mounting of the diodes through the deformable leadsto the common conductor plate and easy accessability to replacement ofthe diode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic electrical diagram of a welding circuit accordingto the invention;

FIG. 2 is a side elevational view in section through a diode assemblyused in the circuit according to the invention;

FIG. 3 is a plan view of the diode assembly seen along lines 3--3 ofFIG. 2; and

FIG. 4 is a plan view of the diode assembly seen along lines 4--4 ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, and to FIG. 1 in particular, there isshown a three-phase power supply system including power supply lines 12,14 and 16 which are connected to Scott-connected transformers 18. Theoutput from the Scott-connected transformers 18 is applied to weldingelectrodes 20 and 22 to weld work pieces 24 and 26 together. A hydraulic(or air) cylinder 28 having a reciprocal rod 30 mounts the electrode 20for movement into contact with and out of contact with the work piece 26during the welding cycle. A control valve 32 controls the flow of fluidthrough lines 34 and 36 to control the flow of fluid to the hydrauliccylinder 28.

A sequence controller 38 is connected to voltage input lines 40 and 42and is adapted to control the operation of valve 32 in a predeterminedtimed sequence through a control line 44.

The transformer 18 has three input terminals 46, 48 and 50 through whichthe transformer is connected to the power supply lines 12, 14 and 16.The input terminals 46, 48 and 50 are respectively connected to primarywindings 52, 56 and 54 which are connected together at a common point58. Windings 52 and 54 have an equal number of turns and the sum of theturns of windings 52 and 54 is greater than the number of turns of thewindings 56. In a standard Scott-connected transformer, the number ofturns on the primary winding 56 is approximately equal to 0.866 timesthe sum of the windings 52 and 54.

Secondary windings 60 are wound to react to current in primary windingsand have output leads 62 and 64 and a center tap output 66. Secondarywindings 68, which are wound to react to the primary windings 52 and 54,have output leads 70 and 72 and a center tap lead 74.

The output lead 64 is connected to a pair of parallel connected diodes78 through a deformable lead 76. If desirable, one or more than twodiodes can be connected to each deformable lead. The output from diodes78 are connected to a common conductor plate 80. In similar fashion,output lead 62 is connected to the common conductor plate 80 through adeformable lead 82 and a pair of parallel connected diodes 84. Outputlead 72 is connected to the common conductor plate 80 through a pair ofparallel diodes 88 and a deformable lead 86. Likewise, the output lead70 is connected to the common conductor plate 80 through a deformablelead 90 and a pair of parallel connected diodes 92. The common conductorplate 80 is connected to the electrode 20 through lead 94. The centertap output leads 66 and 74 are connected to a common conductive plate 96which in turn is connected to the welding electrode 22 through aconnecting lead 98.

A master controller 100 is connected between power supply lines 12 and14 and controls the firing of the transformers. To this end, thecontroller is connected to line 14 through a lead 102 and is connectedto the power supply line 12 through lead 104 and a pair of oppositelybiased silicon control rectifiers (SCR) 106 and 108. The gate of siliconcontrol rectifier 106 is connected to the sequence controller 38 throughcontrol line 110 for control of SCR 106. In like manner, the gate of thesilicon control rectifier 108 is connected to the sequence controller 38through control line 112 for controlling the firing of SCR 108. Theinput terminal 46 is connected to power supply line 12 through a lead114 and through a pair of oppositely biased SCRs 116 and 118. The mastercontroller 100 gates SCRs 116 and 118 through control lines 120 and 122,respectively.

A lead 124 is connected at one end to the input lead 104 and at theother end to the power supply line 14 through an inductance coil 126. Acenter tap lead 127 on the inductance coil 126 is connected to a slavecontrol 128. In addition, the slave control is connected to the powersupply line 16 through lead 129.

The input terminal 48 of the winding 56 is connected to the power supplyline 16 through oppositely biased SCRs 132 and 134 which are gated bythe slave control 128 through control lines 136 and 138 respectively. Alead 140 connects the power supply line 14 directly to the transformerinput terminal 50.

The sequence controller is a conventional controller which controls thesequence of operation of the welding machine. The sequence of operationof the welding machine is conventional and is as follows: During a firsttime interval, known as the squeeze time, the gates to the SCRs 106 and108 are open and no function is performed by the master control 100.However, the sequence controller 38 controls the valve 32 to extend therod 30, thereby bringing the elecrode 20 down into contact with the work26. After the contact between the electrode and the work is complete,the firing cycle takes place. In this cycle, the sequence controller 38will close the SCRs 106 and 108, thereby activating the mastercontroller 100. Rectified input pulses which are a function of the wavepattern in the power supply line 12 produce output signals in leads 120and 122 for gating the SCRs 116 and 118. In this manner, the SCRs 116and 118 are gated to apply rectified voltage pulses during selected timeintervals to the input terminal 46.

The coil 126 with the center tap 127 shifts the phase of the pulses inlead 104 90° to the slave controller 128. The slave controller 128 isadapted to gate the SCRs 132 and 134 responsive to the rectified inputpulses from the center tap 127.

The master control 100 and the slave control 128 are adapted to controlthe firing of the SCRs 116, 118, 132 and 134 in a given sequentialpattern to balance the input from the three-power supply lines. In theembodiment illustrated in the invention, the firing of the SCRs is inthe sequence of 116, 132, 118 and 134. Thus, during the first part ofthe firing cycle, SCR 116 will be gated during the first 90° of the wavepattern of the voltage in power supply line 12. Subsequent thereto, SCR116 is opened and SCR 132 is closed, thereby passing current betweenpower supply line 16 and input terminal 48. Subsequent to the second 90°phase of the wave pattern, SCR 132 is opened by the slave control 128and SCR 118 is closed by the master control 100. After the third 90°period, the master control 100 will open the SCR 118 and the slavecontrol 128 will close the SCR 134. Thus, the input terminals 46 and 48rotate every 90° with respect to input voltage applied thereto. As aresult, alternating current will be generated in the secondary coils 60and 68 and output voltage will be generated at output leads 70, 72, 62and 64. The sequence of operation is lead 70, lead 62, lead 72 and lead64. The outputs are rectified in the diodes 92, 88, 84 and 78 so that aDC current is applied during the firing cycle through lead 94 to theelectrode 20. The electrode 22 provides the ground lead through lead 98for the system.

Subsequent to the firing cycle, the sequence controller 38 opens theSCRs 106 and 108 so that the master controller is closed off. Thetransformer 18 will thus have no power supplied thereto and there willbe no output voltage between leads 94 and 98. The electrodes 20 and 22,however, are maintained in fixed position for a predetermined "holdtime" while the weld solidifies. Thereafter, the sequence controller 38will apply a control signal through control line 44 to the valve 32 tochange the flow of pressure into the cylinder 28. In this manner, therod 30 is withdrawn and the electrode 20 is raised.

Reference is now made to FIGS. 2 through 4 which show the mounting ofthe diode assembly on the transformer. The four secondary output leads64, 62, 72 and 70 are aligned in a common plane. That is to say, each ofthe output leads is formed of a relatively flat configuration with topand bottom surfaces of greater extent than other surfaces, and the topsurfaces of the output leads lie entirely within a common plane. Thedeformable leads 76, 82, 86 and 96 are secured at one end to thesecondary output leads 64, 62, 72 and 70, respectively. Each deformablelead has a thicker portion 142 adjacent to the secondary output leadsand a thinner portion 144 remote from the secondary output leads. Thethicker portion 142 is approximately twice the thickness of portion 144.Wafer diodes 78 are securely positioned between the deformable leads 76,82, 86 and .[.96.]. .Iadd.90 .Iaddend.at one end thereof and the commonconductor plate 80 at the other end thereof. Chill blocks 154 havingcooling water passages 156 are mounted between the diodes 78 and thedeformable leads 76, 82, 86 and .[.96.]. .Iadd.90 .Iaddend.for coolingthe diodes 78. One set of four diodes is positioned beneath the thickerportion 142 of the flexible lead 76 and one set of four diodes ispositioned beneath the thinner portion 144. Thus, the deformable lead 76is tapered proportionally to the distance from the connection thereof tothe secondary output leads of the transformer in order to provide a moreequal resistive path for the two diodes in each set of diodes. Blocks146 and 148 are positioned on top of the flexible lead 76 and springstraps 150 and 152 are positioned on top of the blocks 146 and 148respectively. Spring straps 162 and 164 are positioned beneath thecommon conductor plate 80 in alignment with the spring straps 152 and150 respectively. Bolts 166 mounted within insulated sleeves in boreswhich extend through plates 146, 148, deformable leads 76, chill blocks156, connector plate 80 and through the straps 160 and 162 extendthrough the sleeves to secure the diode assemblies in place between thedeformable leads 76, 82, 86 and .[.96.]. .Iadd.90 .Iaddend. and thecommon plate 80. The spring straps 150, 152, 160 and 164 are springsteel straps which have a slight curvature at the central portion whichbears against the adjacent plates. Thus, a predetermined amount oftension is applied to the bolts through the spring straps. The tensionpermit expansion and contraction of the diodes with varying temperatureconditions while maintaining clamping pressure on the diode assembly.

The deformable leads 76, 82, 86 and .[.96.]. .Iadd.90 .Iaddend.aredesirably made from multiple laminated straps of deformable andconductive material such as copper. The leads are double thickness, forexample 3/4", at the thicker portions (142 for lead 76) compared withthe thinner portions (144 for lead 76). The individual straps are, forexample, 0.010" thick and 2-3 inches wide. The deformability of theleads are important so that all diodes can be mounted to the commonplate without applying unequal pressure to different diodes. The leadsdeform as the diodes are secured in place between the leads and theconductive plate 80. This system of mounting the diodes provides amechanism whereby any defective diode can be easily replaced withoutdisassembly of the entire diode package. .Iadd.This same result of equalpressure on each diode can also be achieved by interchanging theposition of the deformable leads 76, 82, 86 and 90 and the commonconductor plate 80. .Iaddend.

As illustrated in FIGS. 2 and 3, the common conductor plate 80 has aseries of slots 158 extending across the plates between the two sets ofdiodes to further assist in providing a more equal resistive path forthe two diodes in each set of diodes. Further, cooling water passagesextend through the plate 80 to prevent excessive heat buildup thereinand to maintain the diodes relatively cool.

As illustrated in FIG. 2, the common conductor plate 96 is connected tothe secondary output leads 66 and output lead 72 (not visible in FIG.2). The connecting lead 98 is shown secured to the other end of theconductive plate 96.

With the use of the invention, a full 90° conduction time can beachieved in a welding apparatus and the magnitude of the ripple issignificantly lower than that of a full-wave single-phase, directcurrent welding apparatus. Further, with the use of this system, theprimary current will be spread evenly over all three lines. TheScott-connected transformer with the controls according to the inventionis useful for welding aluminum, steel or other such materials. Thissystem is compact in size, less costly to build (because of thesimplified controls) and low on power consumption, because of the 90°conduction cycle and low ripple. Further, expensive and cumbersomereactant coils are not required.

Whereas the invention has been described with reference to a resistancewelding apparatus, the invention can also be used for DC plating and arcwelding apparatus.

Reasonable variation and modification are possible within the scope ofthe foregoing disclosure and drawings without departing from the spiritof the invention which is defined in the accompanying claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. In a welding apparatushaving Scott-connected transformers, three primary input terminals whichare adapted to be connected to a three-phase power supply and foursecondary output leads with which are connected to a first electrodethrough wafer diode assemblies, the transformer further having a centertap output lead from each of the secondary coils, which center tapoutput lead is connected to a second welding electrode, whereby a DCcircuit is made through the first and second electrodes, the improvementcomprising:all of the four secondary output leads formed in a relativelyflat configuration with top and bottom surfaces of greater extent thanother surfaces of said leads, at least the top surfaces of each of saidoutput leads lie entirely in a common plane; deformable leads formed ofmultiple laminated straps of conductive material connected between eachof the output leads and the diodes for firmly supporting the diodes; acommon conductor plate member of relatively flat configuration having asurface of greatest extent lying entirely in a plane parallel to thecommon plane of the output leads; and means clamping the diodeassemblies under pressure between the common conductor plate and thedeformable leads so that the leads deformably respond to any variationin thickness of the diodes as the diodes are secured in place so thatequal pressure can be applied to each diode.
 2. A welding apparatusaccording to claim 1 wherein first and second diodes are providedbetween each deformable lead and the common conductor plate; thedeformable leads have a first thickness between the output leads and apoint spaced therefrom beyond the first diode and a second thicknesssubstantially less than the first thickness between the point and theouter ends of the leads, the second diode being mounted to thedeformable lead at an area which includes the second thickness wherebythe deformable leads are tapered in accordance with the distance fromthe secondary output leads to provide substantially equal resistivepaths for each set of diodes.
 3. A welding apparatus according to claim2 wherein the second thickness is approximately half of the firstthickness.
 4. A welding apparatus according to claim 2 wherein thecommon conductor plate is slotted between the two sets of diodes.
 5. Inan apparatus for converting three-phase alternating current to directcurrent, said apparatus including transformers having Scott-connectedprimary windings with three input terminals for connection to athree-phase power supply, a pair of secondary windings with outputterminals at opposite ends thereof, and a common center tap outputterminal connected to the center of each secondary winding; a commonconductor means for connecting together all of the secondary outputterminals; and means for rectifying the output from each secondaryoutput terminal; the improvement which comprises:control means forregulating the voltage applied to a first and second of the primaryinput terminals such that voltage applied to the first and secondterminals alternates each 90° interval throughout the electrical cyclein such a manner that the current between each output terminal and thecenter tap output in the secondary windings conducts successivelythrough each of the four output terminals during successive 90°intervals of the electrical cycle; the first input terminal beingconnected to one primary winding and the second input terminal beingconnected to the other primary winding; whereby the rectified outputcurrent between the common conductor means and the common center tapoutput terminal is substantially conductive throughout the entireelectrical cycle and the ripple effect in the output is minimized.
 6. Anapparatus according to claim 5 wherein the control means includes firstand second oppositely biased gated rectifying means connected inparallel to the first input terminal, and third and fourth oppositelybiased gated rectifying means connected in parallel to the second inputterminal; and the control means include means for gating the firstrectifying means during a first time interval, means for gating thethird rectifying means during a second time interval, means for gatingthe second rectifying means during a third time interval and means forgating a fourth rectifying means during a fourth time interval.
 7. Anapparatus according to claim 6 wherein the gated rectifying means aresilicon controlled rectifiers.
 8. An apparatus according to claim 7 andfurther comprising deformable leads connected between each of the outputleads;the output terminals are formed in a relatively flat configurationwith top and bottom surfaces of greater extent than other surfaces ofsaid terminals, at least the top surfaces of each of said outputterminals are all positioned entirely in a common plane; a commonconductor plate member of relatively flat configuration having a surfaceof greatest extent lying entirely in a plane parallel to the commonplane of the output terminals; and means mounting the diodes underpressure between the common conductor plate and the deformable leads sothat the leads deformably respond to any variations in thickness of thediodes as the diodes are secured in place so that equal pressure can beapplied to each diode.
 9. An apparatus according to claim 8 whereinfirst and second diodes are connected between each deformable lead andthe common conductor plate member; and the deformable leads have a firstthickness adjacent to the secondary output terminals at the first diodesand a second thickness substantially less than the first thicknessremote from the secondary output terminal at the second diodes;wherebyequal resistive paths are provided for the first and second diodes. 10.An apparatus according to claim 9 wherein the common conductor platemember is slotted between the two sets of diodes, thereby furtherequalizing the resistive paths for each set of diodes.
 11. A weldingapparatus comprising the apparatus according to claim 10 and furthercomprising a first welding electrode electrically coupled to the commonconductor plate member, and a second welding electrode electricallycoupled to the center tap terminals.
 12. In a welding apparatus havingScott-connected transformers with three primary input terminals whichare adapted to be connected to a three-phase power supply and foursecondary output leads which are connected to a first electrode throughdiode assemblies, the transformer further having a center tap outputlead from each of the secondary coils, which lead is connected to asecond welding electrode, whereby a DC circuit is made through the firstand second electrodes, the improvement comprising:at least two of thefour secondary output leads are formed in a relatively flatconfiguration with top and bottom surfaces of greater extent than othersurfaces of said leads, at least the top surfaces of each of the twooutput leads lie entirely in a common plane; deformable leads connectedbetween each of the output leads and the diodes for firmly supportingthe diodes, the deformable leads comprising multiple straps ofconductive material which are laminated together; a common conductorplate member of relatively flat configuration having a surface ofgreatest extent lying entirely in a plane parallel to the common planeof the two output leads; and means clamping the diode assemblies underpressure between the common conductor plate and the deformable leads sothat the leads deformably respond to any variation in thickness of thediodes as the diodes are secured in place so that equal pressure can beapplied to each diode. .Iadd.
 13. In a welding apparatus having atransformer with primary input terminals which are adapted to beconnected to a power supply, at least two secondary output leads whichare connected to a first electrode, the secondary output leads beingformed in a relatively flat configuration with top and bottom surfacesof greater extent than other surfaces of said leads, at least one of thetop and bottom surfaces of said secondary output leads lies entirely ina common plane; a center tap output lead from the transformer connectedto a second electrode whereby an electrical circuit is made between saidfirst and second electrodes; and a diode assembly between the secondaryoutput leads and the first electrode to convert the output of thesecondary output leads to direct current; the improvement in the diodeassembly which comprises: a plurality of diodes each of which having tworelatively flat sides in parallel relationship; a common conductor plateof relatively flat configuration having a surface of greatest extentlying entirely in a plane parallel to the common plane of the secondaryoutput leads and in contact with one flat side of each of said diodes; aplurality of deformable leads, each of said deformable leads being incontact with a second flat side of one of said diodes, each of thedeformable leads comprising multiple straps of conductive material whichare laminated together; and means clamping said diodes between saiddeformable leads and said common conductor plate to apply equal pressureto each of said diodes. .Iaddend..Iadd.
 14. A welding apparatusaccording to claim 13 wherein said deformable leads are each secured toone of said secondary output leads. .Iaddend.